For a seat used for an aircraft, a train, a ship, an automobile, and the like, improvement of various functions such as physique difference absorbency, posture difference absorbency, body movability, and so on are always required in addition to improvement of basic functions such as impact absorbency, vibration absorbency, and so on, in order to supplement these functions and obtain better riding comfort. Further, in recent years, in order to reduce weight of transportation machines such as the automobile from a viewpoint of environmental measures by improving fuel economy, a reduction in weight of the seat used for these transportation machines is desired in addition to the improvement of the various functions described above, and technology to use a thinner and lighter cushioning member or the like is proposed.
For example, from the above viewpoint, the present applicant has proposed a seat structure with sufficient characteristics (a spring characteristic or a damping characteristic) as a cushioning member although light weighted by using a three-dimensional net member (a solid knitted fabric) having a thickness of about several millimeters to several tens or millimeters strained to frames as a tension structure.
However, in order to exhibit sufficiently the characteristics required for the seat or the like of the automobile using such a solid knitted fabric, the various functions such as the vibration absorbency, the physique difference absorbency, and the like are required to serve effectively by disposing a flat-type supporting member composed of a two-dimensional net member or the solid knitted fabric, or a flat-type elastic member called Plumaflex(™) or the like under the solid knitted fabric, and by supporting it with plural metal springs (coil springs) to supplement a feel of a spring and to support a load dispersedly. However, when the plural coil springs are arranged, the solid knitted fabric itself being thin, a seated person may often feel hitting of the coil springs to the solid knitted fabric as a feeling of something foreign. Accordingly, an increase of the number of solid knitted fabric layers or a disposition of another cushioning member such as a urethane member between the solid knitted fabric and the coil springs has been put into practice conventionally as an alleviation measure of the feeling of something foreign. Therefore, if the number of the arranged coil springs that cause the feeling of something foreign can be reduced, or if the coil springs can be arranged to somewhere not likely to be felt as a something foreign, a further reduction in weight can be expected by reducing the number of the solid knitted fabric layers, disappearing of necessity to intercalate another cushioning member such as the urethane member, or the like.
When the urethane member that has been generally used as a cushioning member is used, a thickness of urethane member is usually ensured to be 30 mm or more in order not to make the seated person feel something foreign for items different in stiffness such as a wire, a frame, or the like. However, when a urethane member that is made thinner in thickness and lighter in weight is used, there arises a similar disadvantage as the case of the solid knitted fabric. When another cushioning member such as, for example, a two-dimensional net member (a flat-type spring member) containing elastic yam is used, there also arises a similar disadvantage as the above-mentioned case of the solid knitted fabric since the thickness of the member is thin. When the plural two-dimensional net members (flat-type spring members), solid knitted fabrics, or urethane members are used in combination in arbitrary number, a thinner and a lighter cushioning structure can be attained by reducing the number of the arranged coil springs or changing a place of arrangement.
Furthermore, as for the impact absorbency required for the seat for the transportation machines such as the automobile or the like, a reduction of a rebound or the like of a human body from the seat at the time of collision is particularly required. As a measure for this requirement, such a structure that a collision load received in a longitudinal direction is converted to a rotational moment of inertia force around the tuber of ischium and that a seating angle is so changed that depression of the haunches becomes large is desired.
On the other hand, a standard posture of a person being seated on a chair is a state that the haunches of the person are positioned in the rear of a seat cushion of the chair, and that the waist portion of the person abuts on a seat back of the chair. However, there is a report about a way of being seated called a sacrum posture that the haunches move toward front so that the sacrum portion touches the seat cushion (Bulletin of Japanese Society for the Science of Design, Vol. 48, No. 1 2001, pp. 49 to 56, “Suspension Optimum Characteristic of a Wheel Chair for a Handicapped Person Taking Difference in Physique into Consideration”) as a seating posture of the person, especially when an elderly person or a child having small physique is seated, or when the person is seated for a long time. The same thing can be said of the seat for the transportation machines such as the automobile or the like, a chair for office work or for a theater as well as a wheel chair, and such a development of a seat structure as absorbing a difference in physique and a difference in posture without damaging so much of the impact absorbency or the vibration absorbency and as attaining a comfortable sitting comfort is desired even if the person is seated in such a sacrum posture.
Meanwhile, such a structure that the person can easily stand up when he/she stands up from a seating posture is also desired for any of the seat for the aforementioned transportation machines such as the automobile or the like, or for various chairs or wheel chairs used inside or outside of a residence. Furthermore, a small pressure on a femoral region when working a pedal is also desired for a driver's seat of the automobile.